1. Field:
This disclosure is concerned generally with the field of microbial identification and specifically with a microbial testing device which permits the convenient recognition and differentiation of group D streptococcal bacteria.
2. Prior Art:
Steptococcal bacteria may be conveniently classified according to serological groupings. In a system described by R. Lancefield, the groups are designated by letters of the alphabet. One such group classified according to that system is known as group D streptococci. That group can be conveniently sub-divided into two categories of bacteria, the enterococci and the non-enterococci. See, for example, W. R. Bailey et al., The Streptococci, including the enterococci and anaerobic streptococci, Diagnostic Microbiology, 4th Ed., C. V. Mosby Co., pp 122-3 (1974), and R. R. Facklam, Streptococci, Manual of Clinical Microbiology, 2nd Ed., ASM, pp. 104-6 (1974).
The differentiation of these two categories is of clinical significance as the enterococci are penicillin-resistant whereas the non-enterococci are penicillin-susceptible. Incorrect or delayed identification of these bacteria could lead to improper antibiotic therapy. Therefore, the rapid and accurate recognition and differentiation of group D streptococci is of definite importance to the physician and patient.
Currently, two physiological characteristics are used routinely to recognize and differentiate the group D streptococci. These methods are disclosed in detail in the above references. Briefly, members of the group are recognized by their ability to hydrolyze esculin in the presence of bile (e.g. 4% oxgall). Hydrolysis can be visually recognized by incorporating ferric ions in the bile-esculin medium. As hydrolysis proceeds, the medium changes from brown to black due to the presence of the ferric ions. The enterococci sub-group can be differentiated from the non-enterococci by the growth of the former in a growth medium containing about 6.5% NaCl. A typical growth medium is prepared by incorporating a nutrient such as dextrose and a pH sensitive color indicator (e.g. bromcresol purple) into the high NaCl medium. As growth proceeds, the dextrose is converted to by-products which cause a pH (and color) change in the medium.
It is well known that bacterial identification systems can be based on devices which provide a variety of selected growth media each of which, because of its constituents, is capable of indicating the presence or absence of a given characteristic. Thus, by incubating a sample of a microbe on a variety of selected media, noting such features as color change, gas evolution, etc., and then comparing those features with those obtained with known organisms, it is possible to identify the sample microbe. See, for example U.S. Pat. No. 3,830,703 (Enteric Bacilli Differentiation) and U.S. Pat. No. 3,830,702 (Bacteriological Media Tube) both of which are concerned with the use of constricted test tubes to contain two or more bacterial identification growth media.
We have now found that by carefully modifying known techniques for identifying group D streptococci and by carefully combining those features with the identification device disclosed in U.S. Pat. No. 3,830,702, there is made available a new device which makes possible an inexpensive, simple, and accurate method for the relatively rapid recognition and differentiation of group D streptococci. Details of our device and preferred ways of making and using it are described herein.